I was about to say the same as Issey. It can also happen with female cats if a bit of the ovary is left behind, they then still come into season and will be pursued by toms, and a second op is the only answer in this case.

he might have had a bit of his testicles left behind when they neutered him so he will still be producing testosterone, have seen this happen a few times with cats at the rescue i work at, the only solution for this is sorry vets and a hormone test to confirm it

Can anyone advise.My two year old male cat was neutered about three months so but still humps like mad.He particularly enjoys humping my faux fur blanket! There has been no reduction and its getting a bit ridiculous. He has also taken to humping DH's feet during the night.Any thoughts?